The present invention relates to an electron beam pattern line width measurement system which measures the line width of a fine pattern such as wiring formed on the surface of a semiconductor substrate, by the use of an electron beam. More particularly, it relates to an electron beam pattern line width measurement system which is well-suited to measure the line width of a pattern on a sample surface made of a plurality of different materials.
A prior-art example which uses a electron beam to detect the amount of secondary electrons and to measure the line width of a pattern, is described in "Ishida et al; Simulation of electron signal waveform on microdimensional line width measurement, 1980, Proceedings of the 41st Autumn Meeting of the Japan Society of Applied Physics, page 324, 17a-F-2". This example evaluates the line width of the pattern of a W (tungsten) gate 24 formed on an Si (silicon) substrate 23 as shown in FIG. 1, under the assumption that each signal corresponding to an edge portion 26, in a secondary electron signal 25 obtained by scanning an electron beam on the surface of the substrate, corresponds to the edge shape of the W gate 24. In the Ishida et al literature, the line width of the pattern is determined from only the signal corresponding to the amount of secondary electrons generated from the sample. In the case of FIG. 1, the W gate 24 is thick enough to afford a good edge shape, and the sectional shape of the sample substrate is known, so that the edge signals could be read from the secondary electron signal. However, it is not always the case that the conditions mentioned above are satisfied. Rather, it is common that there is no considerable difference between a secondary electron signal level corresponding to an Si substrate and a signal level corresponding to a W gate when the W gate is thinned more with rise in the integration density of an electron device. Moreover, in a nondestructive sample measurement, a sectional structure is usually unclear. In such cases, it is difficult to separate the edge signals from the secondary electron signal. The upper part 21 of FIG. 1 shows the sectional structure of the substrate, while the lower part 22 is a graph showing the amounts of secondary electrons versus the various positions of the sectional structure shown in the upper part 21.
Meanwhile, it is known that information specific to a substance is contained in the energy distribution of secondary electrons [refer to, for example, "Ogawa et al; A measurement of secondary electrons by Auger microprobe, 1984, Proceedings of the 31st Spring Meeting of the Japan Society of Applied Physics and of the Related Societies, page 291, 29p-x-10"]. It is known that, in general, the secondary electron energy distribution of insulators including semiconductors and the secondary electron energy distribution of metals exhibit a distinction as shown in FIG. 2 [refer to P. R. Thornton; "Scanning Electron Microscopy", pp. 104-105, 1968, published by Chapman and Hall Ltd., in London, Great Britain]. That is, the secondary electron energy distribution of a semiconductor is narrower than that of a metal, and the secondary electron energy of the semiconductor at a position 35 at which the secondary electron emission yield .sigma. becomes the maximum is 2.5-5 eV, which are smaller than that of the metal being 5-10 eV at the corresponding position 36. In FIG. 2, curves 31, 32, 33 and 34 correspond to the secondary electrons of insulator materials encompassing semiconductors, the secondary electrons of metals, the reflected electrons of the insulator materials encompassing the semiconductors, and the reflected electrons of the metals, respectively.